Powder metallurgy is a common manufacturing method used to produce components of high quality for applications such as engines. Powder metallurgy is often employed in the manufacture of engine components because it is economical, flexible, and can produce a finished part that requires less machining or secondary processing than other methods of forming components. Powder metallurgy allows for a component to be formed of a wide variety of alloys, composites, and other materials to provide the finished component with desirable characteristics. Powder metallurgy is well suited to manufacture parts of a wide range of sizes and shapes. Also, powder metallurgy can reliably produce parts with consistent dimensions and advantageous physical properties.
Referring to FIG. 1, a process chart for the conventional powder metallurgical component forming process 30 is shown. First, the metal powders 32 that comprise the component are provided. Often, lubricants are added to the metal powders to decrease the wear of pressing machinery. Next, the base powders are mixed 34 to form a homogenous mixture. The finished part is ultimately a homogeneous alloy of the constituent metal powders.
A mold or die is then filled 36 with the mixed powders. The die, when closed, has an internal cavity somewhat similar in shape to the final part. The powder is compressed 38 within the die to form a so-called “green part.” The compaction 38 is usually performed at room temperature and at pressures, for example, in the range of 30-50 tons per square inch. The green part, also referred to as a “green compact,” has the desired size and shape for the next operation when ejected from the die. After compaction 38, the green part has sufficient strength for further processing.
The green part is subjected to a sintering process 40. A variety of secondary operations 42 may be performed on the part after sintering 40, depending on its intended use, the process yielding a finished part 44.
Generally, sintering 40 involves subjecting the green part to a temperature, for example, of 70-90% of the melting point of the metal or alloy comprising the green part. The variables of temperature, time, and atmosphere are controlled in the furnace to produce a sintered part having improved strength due to bonding or alloying of the metal particles. The sintering process 40 generally comprises three basic steps conducted in a sintering furnace: burnoff 46, sinter 48, and cooling 50. Continuous-type sintering furnaces are commonly used to perform these steps. The burnoff chamber is used to volatize the lubricants used in forming green part 46. The high-temperature chamber performs the actual sintering 48. The cooling chamber cools the sintered part prior to handling 50.
The parts that exit the sintering furnace 40 after cooling 50 may be considered complete. Alternatively, they may undergo one or more secondary operations 42. Secondary operations include, for example, re-pressing (forging) the component 52, machining 54, tumbling 56, and joining the component with additional components 58 as part of an overall assembly. The secondary operations 42 may also include the impregnation of oils or lubricants 60 into the part for conveying self-lubricating properties. The sintered component may also undergo heat treatment 62 to provide certain characteristics and properties to the component, such as strength. Those skilled in the art will recognize that other secondary operations may be performed. The secondary operations 42 may be performed individually or in combination with other secondary operations. Once all the secondary operations 42 are performed, the component or part 44 is finished.
U.S. Pat. Nos. 5,303,468, 5,195,398, and 3,748,925 disclose crankshafts for use in an internal combustion engine.
FIG. 2 illustrates the internal detail of a conventional internal combustion engine to illustrate the use of a crankshaft 72. A connecting rod 64 is pivotally connected to a piston 66 and the crankshaft 72. The connecting rod 64 is connected to the crankshaft 72 at a large or crank end 76. The large end 76 of the rod 64 receives a shaft portion (“crank pin”) 78 of the crankshaft 72. The connecting rod 64 is further connected to a piston 66 at a small or piston end 70 of the rod 64. The crankshaft 72 comprises a counterweight 74 disposed between the crankpins 78.
Referring to FIG. 3 and FIG. 4, a conventional crankshaft 72, manufactured according to conventional methods, is shown. Crankshaft 72 comprises a longitudinally extending body 83 between a first end 80 and a second end 82. The body 83 defines an axis or rotation 84 for the crankshaft, when rotating in the engine. A main journal 86 is provided at each of the first end 80 and second end 82 for supporting the shaft 72 in the engine block. The body 83 includes a plurality of bearing journals 88, crank pin journals 90 and counterweights 74.
The mass of the crank pin journals 90 when coupled with a connecting rod 64 defines an offset balance axis 92. The balance axis 92 is the axis of rotation through which the forces generated by rotation of the shaft and connecting rod assembly are balanced. The axis of rotation 84 is offset from the axis of balance 92. The offset creates a moment when the crankshaft is rotating. The moment is undesirable because it increases loading on the shaft bearings and minimizes oil film thickness between the journals of the crankshaft and their respective bearings. This limits the load carrying capacity of the main journals.
A conventional solution is to provide a plurality of counterweights 78 to the shaft 72 to shift the axis of balance 92 towards the axis of rotation 84. Ideally, the counterweights 78 are located 180 degrees opposite each crankpin journal 90. Such configuration results in an undesirably large crankshaft 72. Engine designers are constantly striving to minimize engine size and increase engine efficiency. Larger crankshafts necessitate larger engine size. Moreover, larger crankshafts also increase the rotational inertia of the engine, thereby reducing efficiency.
U.S. Pat. No. 5,195,398 discloses one method of addressing the balance versus crankshaft size issue. This patent discloses offsetting one or more of the counterweights with respect to the crankpin journals to form an asymmetric counterweight configuration. The asymmetric arrangement strikes a balance between oil film thickness, crankshaft mass and packaging constraints. The asymmetric design, however, suffers from packaging limitations. The design also has the same disadvantages present in the unbalanced crankshaft and connecting rod assembly, albeit to a reduced degree.
Therefore, there is a need for a method of providing crankshaft that minimizes costs while providing for adequate balance.